✨(frontend) allow hiding the register link

[sampaccoud]

Purpose

Some identity federations to which Magnify will be connected do not allow registration. We need to hide the "register" link in this case.

Proposal

• [x] In backend, add an environment variable setting to allow hiding the register link: SHOW_REGISTER_LINK
• [x] In frontend, hide the register link if the setting returned by the backend asks so
• [x] In frontend, hide the list of rooms when the user is not logged-in and the register link is disabled

Fixes https://github.com/openfun/jitsi-magnify/issues/171 Fixes https://github.com/openfun/jitsi-magnify/issues/172

[jbpenrath]

An entry in the CHANGELOG is missing